U.S. patent application number 11/188110 was filed with the patent office on 2007-02-08 for digital accelerometer.
Invention is credited to Nebojsa Vrcelj.
Application Number | 20070028689 11/188110 |
Document ID | / |
Family ID | 37716421 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070028689 |
Kind Code |
A1 |
Vrcelj; Nebojsa |
February 8, 2007 |
Digital accelerometer
Abstract
A force-balance accelerometer having a pick-off coil responsive
to displacement of a seismic mass from a balance position for
providing an output corresponding to the displacement, includes a
digital signal processor including tow pulse width modulation
generators for converting the output of the pick-up coil to a
digital signal, and a torque coil responsive to the digital signal
for rebalancing the seismic mass by restoring the mass to the
balance position. The processor outputs the digital signal as first
and second PWM signals, which control the digital signal.
Inventors: |
Vrcelj; Nebojsa; (Vancouver,
CA) |
Correspondence
Address: |
LONG AND CAMERON
SUITE 1401 - 1166 ALBERNI STREET
VANCOUVER
BC
V6E 3Z3
CA
|
Family ID: |
37716421 |
Appl. No.: |
11/188110 |
Filed: |
July 25, 2005 |
Current U.S.
Class: |
73/514.17 ;
341/145; 73/514.18 |
Current CPC
Class: |
G01P 15/11 20130101;
G01P 15/132 20130101 |
Class at
Publication: |
073/514.17 ;
073/514.18; 341/145 |
International
Class: |
G01P 15/13 20070101
G01P015/13; H03M 1/68 20070101 H03M001/68 |
Claims
1. An accelerometer, comprising: a movable seismic mass; a pick-off
coil responsive to displacement of the seismic mass from a balance
position for providing an output corresponding to the displacement;
means for converting the output of the pick-up coil to a digital
signal; the means for converting comprising two pulse width
modulation generators; and a torque coil responsive to the digital
signal for rebalancing the seismic mass by restoring the mass to
the balance position.
2. The force-balance accelerometer as claimed in claim 1, wherein
the pulse width modulation generators output the digital signal as
first and second PWM signals.
3. The force-balance accelerometer as claimed in claim 2, including
switches controlled by the first and the second PWM signals,
respectively, for applying electrical signals to the torque
coil.
4. The force-balance accelerometer as claimed in claim 3, wherein
one of the electrical signals includes data representing a preload
control voltage.
5. The force-balance accelerometer as claimed in claim 4, wherein
another one of the electrical signals includes data representing a
compensation control voltage for compensating the effect of the
preload control voltage in the torque coil.
6. The force-balance accelerometer as claimed in claim 3, wherein
the first PWM signal includes means for cancelling energy delivered
to the torque coil when activating the switching means.
7. The force-balance accelerometer as claimed in claim 1, wherein
the accelerometer further includes means for outputting the digital
signal as a PWM signal, a switching means for applying electrical
signals to the torque coil, said switching means being responsive
to the PWM signal, wherein the PWM signal comprises means for
cancelling energy delivered to the torque coil when activating the
switching means.
8. The force-balance accelerometer as claimed in claim 2, wherein
the first PWM signal comprises a first duty cycle corresponding to
a most significant word of the digital signal and the second PWM
signal comprises a second duty cycle corresponding to a least
significant word of the digital signal.
9. The force-balance accelerometer as claimed in claim 1, wherein
the means for converting the output of the pick-off coil to a
digital signal comprises an analog-to-digital converter, the
analog-to-digital converter being responsive to the output of the
pick-off coil.
10. The force-balance accelerometer as claimed in claim 1, wherein
the means for converting the output of the pick-up coil to a
digital signal comprises a feedback network, the feedback network
being responsive to the output of the pick-off coil and providing
the digital signal.
11. The force-balance accelerometer as claimed in claim 1, wherein
the means for converting the output of the pick-off coil to a
digital signal employ a computer program executed on the processor
and responsive to the output of the pick-off coil to provide the
digital signal.
12. The force-balance accelerometer as claimed in claim 3,
including a multiplexor responsive to the first and the second PWM
signals and providing switch control signals for the switches.
13. A force-balance type accelerometer having a negative feedback
network operable to rebalance a mass accelerated by an external
force and one or more feedback paths, each of the feedback paths
providing an analog-to-digital conversion of a system variable that
is monitored by the negative feedback network for changes in a
parametric value, the negative feedback network compensating for
the changes in the parametric values thereby calibrating the
accelerometer.
14. The force-balance type accelerometer of claim 13, wherein the
system variables are chosen from the group consisting of
temperature, voltage of a torque coil, power supply voltage and
amplifier offset of a position detector.
15. A high resolution digital to analog convertor for converting a
digital signal to an analog signal comprising: first and second PWM
generators providing first and second PWM signals respectively,
said first PWM generator being controlled by a most significant
word of the digital signal, and said second PWM generator being
controlled by a least significant word of the digital signal; first
and second switches controlled by the first and the second PWM
signals, respectively; and first and second current sources having
first and second currents, respectively, the first and second
currents being switched by the first and the second switches
thereby providing first and second switched currents, respectively,
whereby the analog signal is obtained by the combination of the
first and second switched currents.
16. The high resolution digital to analog converter as claimed in
claim 15, wherein the first current is greater in magnitude than
the second current.
17. The high resolution digital to analog converter as claimed in
claim 15, wherein the first and second switches are each one of a
solid state switch or a mechanical switch.
18. The high resolution digital to analog converter as claimed in
claim 15, wherein the first and the second current sources comprise
resistors.
19. A method of providing a high resolution DAC for converting a
digital signal to an analog signal comprising the steps of:
providing a first PWM signal having a duty cycle corresponding to a
most significant word of the digital signal; providing a second PWM
signal having a duty cycle corresponding to a least significant
word of the digital signal; providing a first electrical signal,
said first electrical signal having a duty cycle equal to the duty
cycle of the first PWM signal; providing a second electrical
signal, said second electrical signal having a duty cycle equal to
the duty cycle of the second PWM signal; and combining the first
the second electrical signals to provide the analog signal.
20. The method of providing a high resolution DAC as claimed in
claim 20, wherein the step of providing the first PWM signal
includes the step of controlling a first PWM generator with the
most significant word of the digital signal.
21. The method of providing a high resolution DAC as claimed in
claim 20, wherein the step of providing the first electrical signal
includes the step of controlling a switch with the first PWM
signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to force-balance
accelerometers and, more particularly, to feedback loops for the
pick-off coils of force-balance accelerators.
[0003] 2. Description of the Related Art
[0004] In U.S. Pat. No. 4,088,027, issued May 9, 1978 to Hernandez
et al., there is disclosed a force balance servo accelerometer
including a D'Arsonval type mechanism for rebalancing, between a
pair of sensing coils, a seismic mass moved by acceleration. The
D'Arsonval type mechanism comprises a pair of suspension beams
mounted in parallel planes in a liquid filled cylindrical housing.
A pair of axially aligned taut wires support a torque coil between
the suspension beams. The coil surrounds a permanent magnet fixedly
mounted in the housing. An arm extending outwardly from the coil,
transverse to the axis of the taut wires, supports the seismic mass
between the sensing coils, which are mounted in the housing. The
sensing coils form two arms of a bridge circuit energized by an
oscillator connected across one pair of opposing terminals of the
bridge. The signal developed across the other pair of opposing
terminals is applied to a differential amplifier. The resultant
difference signal is sine wave multiplied with the output of the
oscillator in a quadrature detector. The output of the quadrature
detector, which is related to the acceleration of the seismic mass,
is applied to the coil of the D'Arsonval mechanism to rebalance the
seismic mass. A bellows mounted on one end of the housing allows
the liquid in the housing to expand and contract as the temperature
of the environment in which the D'Arsonval type mechanism is
located changes.
[0005] In U.S. Pat. No. 4,315,434, issued Feb. 16, 1982 to Marcus
R. Eastman, there is disclosed an accelerometer, the output of
which is sent to a pulse width modulating digitizing circuitry
comprising a comparator to generate a PWM signal, a flip-flop
steering circuit, an "H" switch to toggle a torquer constant
current either positive or negative, and an AND gate to gate clock
pulses for the output.
[0006] The accelerometer taught in the afore-said U.S. Pat. No.
4,315,434 has the disadvantage of a continuous torquer current,
i.e. a bipolar current through a torque coil, and therefore a large
power consumption. Furthermore, with reference to FIG. 2 of
Eastman, it is clear that the feedback loop from a pick-off coil
(14) to a torque coil (24) consists entirely of analog components,
with exception of the flip-flop (22). Therefore, the accelerometer
of the Eastman reference is very susceptible to aging and drift of
components, temperature, and noise sources. The Eastman reference
does not provide a technique to compensate for the above effects on
the components, and therefore must be returned to the manufacturer
for calibration.
[0007] An additional disadvantage of that accelerometer is that it
does not provide a measured value for the acceleration, but
instead, provides a digital pulse train that is proportional to the
sensed acceleration (see column 2, line 6). Clearly, additional
circuitry is required in order to obtain a measurement of the
acceleration.
[0008] That prior accelerometer has the disadvantage of including
much of the feedback loop of the accelerometer in the analog
domain, which makes it overly sensitive to temperature,
manufacturing and aging tolerances.
BRIEF SUMMARY OF THE INVENTION
[0009] In a first aspect of the present invention, there is
provided a force-balance accelerometer comprising a movable seismic
mass, a pick-off coil responsive to displacement of the seismic
mass from a balance position for providing an output corresponding
to the displacement, means for converting the output of the pick-up
coil to a digital signal, and a torque coil responsive to the
digital signal for rebalancing the seismic mass by restoring the
mass to the balance position.
[0010] In a second aspect of the present invention, there is
provided a force-balance accelerometer comprising a movable seismic
mass, a pick-off coil responsive to displacement of the seismic
mass from a balance position for providing an output corresponding
to the displacement, means for converting the output of the pick-up
coil to a digital signal, a torque coil responsive to the digital
signal for rebalancing the seismic mass by restoring the mass to
the balance position, and means for outputting the digital signal
as first and second PWM signals.
[0011] In a third aspect of the present invention, there is
provided a force-balance type accelerometer having a negative
feedback network and one or more feedback paths. The negative
feedback network is operable to rebalance a mass accelerated by an
external force. Each of the feedback paths provides an
analog-to-digital conversion of a system variable that is monitored
by the negative feedback network for changes in a parametric value.
The negative feedback network compensates for the changes in the
parametric values, thereby calibrating the accelerometer.
[0012] In a fourth aspect of the present invention, there is
provided a high resolution digital to analog convertor for
converting a digital signal to an analog signal that comprises
first and second low resolution PWM generators, first and second
switches and first and second current sources. The first and second
PWM generators provide first and second PWM signals respectively.
The first PWM generator is controlled by a most significant word of
the digital signal, and the second PWM generator is controlled by a
least significant word of the digital signal. The first and second
switches are controlled by the first and the second PWM signals
respectively. The first and second current sources have first and
second currents respectively. The first and second currents are
switched by the first and the second switches, thereby providing
first and second switched currents respectively. The analog signal
is obtained by the combination of the first and second switched
currents.
[0013] In a fifth aspect of the present invention, there is
provided a method of providing a high resolution DAC for converting
a digital signal to an analog signal comprising the steps of
providing a first PWM signal having a duty cycle corresponding to
the most significant word of the digital signal; providing a second
PWM signal having a duty cycle corresponding to the least
significant word of the digital signal; providing a first
electrical signal, said first electrical signal having a duty cycle
equal to the duty cycle of the first PWM signal; providing a second
electrical signal, said second electrical signal having a duty
cycle equal to the duty cycle of the second PWM signal; and
combining the first the second electrical signals to provide the
analog signal.
[0014] The present invention is preferably embodied as a seismic
motion sensor and, more particularly, as a broadband type of
force-balance accelerometer used to measure signals in a wide range
of amplitudes and frequencies, the performance of which strongly
depends on the performance of a feedback loop.
[0015] The feedback loop is digital with a minimum number of
components, designed using primarily integrated circuits, and
relying on an optimized way of current switching to reduce the
required space sand to improve temperature stability and noise
immunity, in comparison to prior art accelerometers using an analog
feedback loop, while maintaining good dynamic performance.
[0016] State of the art pulse width modulation (PWM) generators in
digital signal processors are limited by fifteen bits of
resolution, and therefore the dynamic range of a digital-to-analog
converter comprising one of these low resolution PWM generators is
limited to fifteen bits of resolution.
[0017] The technique of the present invention overcomes this
limitation by providing a digital-to-analog converter of
substantially greater dynamic range by combining two or more low
resolution PWM generators that, in combination with additional
circuitry, theoretically provide any number of bits of accuracy.
For example, the preferred embodiment of the invention illustrates
a twenty four bit, high resolution digital-to-analog converter, the
output of which is applied to the torque coil, which is achieved by
combining two low resolution PWM generators that produce respective
PWM signals.
[0018] The PWM signals from respective PWM generators each
contribute to the high resolution output by varying its respective
duty cycle and, therefore, an electrical signal, i.e. a current
and/or voltage signal, applied to the torque coil. The current
flowing through the torque coil is discrete and, in contrast to the
well known PWM patterns, also includes amplitude control. The
amplitude of the current flowing through the torque coil is
controlled by providing a feedback path for the voltage signal
across the torque coil. This provides an additional level of
control of energy delivered to the load, i.e. the torque coil.
[0019] The primary purpose of the pulse amplitude control is to
compensate for the electrical drifts in the torque coil, caused,
for example, by aging or temperature change, thus allowing low
frequency components to be differentiated from the electrical
drifts and promoting proper distribution of the currents related to
each PWM generator. The dynamic method of the PWM signal pulse
amplitude control reduces the amount of noise generated by current
switching, for example, it reduces power supply noise.
[0020] The frequency and period of the pulse stream does not affect
the delivered energy, which depends only on the duty cycle.
Moreover, using the symmetric driving pattern described herein,
linearity of the system is dramatically improved by removing the
effects of transient processes, making the delivered energy
directly proportional to the pulse width. The amplitude control is
a kind of calibration that is performed in run time without
interruption of the data acquisition process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The invention will be more readily understood from the
following description of an embodiment thereof given, by way of
example only, with reference to the accompanying drawings, in
which:--
[0022] FIGS. 1A, 1B & 1C show block diagram views according to
one embodiment of the present invention;
[0023] FIG. 2 shows a schematic view of elements of an
accelerometer of the embodiment of FIGS. 1A, 1B & 1C;
[0024] FIG. 3 shows a block diagram view of the accelerometer and a
negative feedback loop of the embodiment of FIGS. 1A, 1B &
1C;
[0025] FIGS. 4A & 4B show diagrammatic views of a digital to
analog converter of the embodiment of FIGS. 1A, 1B & 1C;
[0026] FIG. 5 shows a diagrammatic view of the resolution of the
digital-to-analog converter of the embodiment of FIGS. 1A, 1B &
1C;
[0027] FIG. 6 shows a graphical view of electrical energy delivered
to a torque coil of the embodiment of FIGS. 1A, 1B & 1C;
[0028] FIGS. 7A & 7B shows a graphical views of pulse width
modulated signals of the embodiment of FIGS. 1A, 1B & 1C;
and
[0029] FIGS. 8A, 8B, 8C & 8D show a schematic view of the
embodiment of FIGS. 1A, 1B & 1C.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0030] Referring to the figures and firstly to FIGS. 1A, 1B, 1C
& 2, there is a force balance-type accelerometer indicated
generally by reference numeral 11, which has a fluid-damped
metal-flexure suspension. There is a pick-off coil 10 and an
unbalanced weight 13 attached to a shaft 15 forming a pendulum,
which is suspended in a permanent magnetic field and is free to
move in one degree of freedom only. When subjected to acceleration,
this pendulum moves with respect to the accelerometer frame.
[0031] The pick-off coil 10 provides an angular position signal 12,
e.g. a voltage signal, which is applied to a position detector 14,
which is a differential position sensor in this example. The
position detector 14 electronically senses the movement of the
pick-off coil 10 and provides an analog output signal 16. The
output signal 16 of the position detector 14 is applied to a high
resolution analog-to-digital converter (ADC) 18, which provides a
twenty four bit digital signal 20 representative of the angular
position signal 12. The ADC 18 samples the output signal 16 once
every sampling period T.sub.SAMPLE (FIGS. 7A & 7B).
[0032] The digital signal 20 is inputted into a digital signal
processor 22 over a communication bus 23, where it is used in an
algorithm to provide a feedback signal 36 (FIGS. 4A & 4B) to
the pick-off coil 10. The processor 22 comprises a first pulse
width modulation (PWM) generator 24, a second PWM generator 26 and
an arithmetic logic unit (ALU) 28. The processor 22 also comprises
a memory (not shown), which stores instructions representative of
the algorithm, and which are executed by the processor 22.
[0033] The ALU 28, when configured by the instructions of the
algorithm, comprises a multiplier 30 and a digital feedback network
32, which is a proportional-integral-derivative filter in this
embodiment. The digital feedback network 32 is part of a negative
feedback loop that controls the angular position of the pick-off
coil 10, and has a transfer function optimized for this
purpose.
[0034] Transfer functions in negative feedback loops are well
developed in the art, and the particular transfer function used in
the digital feedback network 32 is a design decision. In other
embodiments, the digital feedback network 32 can be a fuzzy logic
network that adapts the parameters of the transfer function of the
network 32 depending on the angular position signal 12, or can
comprise the common regulator algorithm, i.e. the Kalman regulator,
which is a predictive algorithm that attempts to differentiate
noise from the signal 12.
[0035] The algorithm is a feedback algorithm which accepts the
digital signal 20 and provides the digital feedback signal 34,
which is a twenty four bit word as shown in FIGS. 4A & 4B. The
digital feedback signal 34 is representative of a duty cycle for a
feedback signal 36 that is applied to a torque coil 38. The
feedback signal 36 is a high resolution, pulse width modulated
signal which also varies in amplitude. The unbalanced weight 13 is
brought back into balance by applying the correct feedback signal
36 to the torque coil 38.
[0036] The algorithm in the processor 22 calculates the required
twenty four bit digital feedback signal 34, i.e. the duty cycle
value, and splits it into two segments according to the following
procedure. The digital feedback signal 34 comprises a most
significant word (MSW) and a least significant word (LSW). The MSW
comprises the top fourteen bits of the twenty four bit digital
feedback signal 34, and the LSW comprises the lower ten bits of the
twenty four bit digital feedback signal 34. If the MSW is less than
one thousand and twenty four (1024), then the MSW is multiplied by
sixteen, to extend it up to a fourteen bit value, providing an MSW
segment 40. If the MSW is greater than or equal to one thousand and
twenty four (1024) then its value is unmodified and provides an MSW
Full Scale segment 42. The LSW provides an LSW segment 44.
[0037] The MSW segment 40 and the MSW Full Scale segment 42 are
used to set the duty cycle of a first PWM signal 46 (FIGS. 1A, 1B
& 1C), which is a digital signal, from the PWM generator 24. In
each of the sampling periods T.sub.SAMPLE, only one of the segments
40 or 42 is used, i.e. the segments 40 and 42 are mutually
exclusive, since the MSW of the digital feedback signal 34 is
constant for each sampling period T.sub.SAMPLE. The LSW segment 44
is used to set the duty cycle of a second PWM signal 48, which is a
digital signal, from the PWM generator 26.
[0038] The processor 22 provides the first and second PWM signals
46 and 48 and first and second control signals 52 and 54 to a
multiplexor 50. The first control signal 52 indicates whether the
duty cycle of the first PWM signal 46 is controlled by the MSW
segment 40 or the MSW Full Scale segment 42. The second control
signal 54 controls the direction of the feedback signal 36 through
the torque coil 38, as will be explained in more detail below.
[0039] The multiplexor 50 provides eight switch control output
signals 56, 58, 60, 62, 64, 66, 68 and 70 that control switches
SW.sub.1-SW.sub.8, respectively. The switches SW.sub.1-SW.sub.8 are
preferably solid state switches, such as a MOSFET device, but can
be other types of switches, such as mechanical switches.
[0040] The switch control signals 56 and 62 correspond to the first
PWM signal 46 when the MSW full scale segment 42 controls the duty
cycle of the signal 46. The switch control signals 58 and 64
correspond to the first PWM signal 46 when the MSW segment 40
controls the duty cycle of the signal 46. The switch control
signals 60 and 66 correspond to the second PWM signal 48, whose
duty cycle is controlled by the LSW segment 44.
[0041] The switch control signals 68 and 70 determine the direction
of the feedback signal 36 in the torque coil 38. The switches
SW1-SW8 and the switch control signals 56-70 together establish six
channels 72-82 of electrical energy, i.e. current and/or voltage,
which are selectively switched on to create the feedback signal 36.
Table 1 below indicates which of the switches SW1-SW8 are on for
each of the channels 72-82. The column labelled "Direction"
indicates the direction the current in the respective channel flows
through the torque coil 38. The quiescent current in each of
channels 72, 74, 76, 78, 80 and 82 is substantially determined by
resistors R1, R2, R3, R4, R5 and R6, respectively, in this
example.
[0042] The multiplexer 50 selects which of the channels 72-82 are
to be driven. For each of the sampling periods T.sub.SAMPLE, there
are four channel combinations that may be selected. A first
combination includes channels 72 and 76, a second combination
includes channels 74 and 76, a third combination includes channels
78 and 82 and a fourth combination includes channels 80 and 82. In
each of the combinations mentioned above, each of the channels
72-82 is selected mutually exclusively of the other channel, in
this example. TABLE-US-00001 TABLE 1 Channel SW1 SW2 SW3 SW4 SW5
SW6 SW7 SW8 Direction 72 on off off off off off off on positive 74
off on off off off off off on positive 76 off off on off off off
off on positive 78 off off off on off off on off negative 80 off
off off off on off on off negative 82 off off off off off on on off
negative
[0043] The first combination corresponds to the PWM signal 46,
controlled by the MSW full scale segment 42, and the PWM signal 48
providing positive electrical energy, i.e. current and/or voltage,
to the torque coil 38. The second combination corresponds to the
PWM signal 46, controlled by the MSW segment 40, and the PWM signal
48 providing positive electrical energy to the torque coil 38.
[0044] The third combination corresponds to the PWM signal 46,
controlled by the MSW full scale segment 42, and the PWM signal 48
providing negative electrical energy, i.e. current and/or voltage,
to the torque coil 38. The fourth case corresponds to the PWM
signal 46, controlled by the MSW segment 40, and the PWM signal 48
providing negative currents to the torque coil 38.
[0045] The channels 74 and 80, which correspond to positive and
negative current contributions of the PWM signal 46 controlled by
the MSW full scale segment 42, are used to extend the pulse
duration and to decrease the pulse amplitude to deliver the same
amount of energy in order to reduce the overall amplitude of the
current being switched, and, therefore, reduce the amount of noise
caused by the switching.
[0046] The positive electrical signal of the channel 72, which is
applied to the torque coil 38, corresponds to the negative
electrical signal of the channel 78, which is also applied to the
coil 38. The electrical signals of the channels 72 and 78 are
substantially equal in magnitude and opposite in direction. In the
present embodiment, in order to achieve substantially equal
magnitudes of the electrical signals of the channels 72 and 78,
resistors R1 and R4 are preferably well matched in value.
[0047] In a similar manner, resistors R2 and R5 are preferably well
matched in value so that the electrical signals of the channels 74
and 80 are substantially equal in magnitude, and resistors R3 and
R6 are preferably well matched in value so that the electrical
signals of the channels 76 and 82 are substantially equal in
magnitude. In other embodiments, it is possible to use active
current sources instead of resistors in order to achieve highly
matched electrical signals in respective corresponding channels 72
& 78, 74 & 80 and 76 & 82.
[0048] The current signals of the channels 72 and 78, corresponding
to the MSW full scale segment 42, are greater in magnitude than the
current signals of the channels 76 and 82, which correspond to the
LSW segment 44. This aspect allows the two low resolution PWM
generators 24 and 26, which are fifteen bit PWM generators in this
example, to provide a digital-to-analog convertor of twenty four
bit resolution, and therefore increased dynamic range. This is
diagrammatically illustrated in FIG. 5.
[0049] The current signals of the channels 74 and 80, which
correspond to the MSW segment 40, are also greater in magnitude
than the current signals of the channels 76 and 82, which
correspond to the LSW segment 44, but less in magnitude than the
current signals of the channels 72 and 78, which correspond to the
MSW full scale segment 42. However, since the MSW segment 40 is
derived by multiplying the MSW of the digital feedback signal 34 by
sixteen, the time base of the current signals of the channels 74
and 78 are extended. The current signals of the channels 74 and 78
are therefore applied over a greater period of time, and the
effective resolution is therefore equivalent to the MSW full scale
segment. This is diagrammatically illustrated in FIG. 6.
[0050] The acceleration is calculated based on the digital feedback
signal 34 and the electrical signals of the channels 72-82, and in
particular the current signals of the channels, since the
acceleration is proportional to the total current through the
torque coil 38. The total current through the torque coil is
proportional to the current signals of each the channels 72-82 and
the respective duty cycles of those current signals. An equation
for calculating total current through the torque coil 38 is
illustrated in FIGS. 4A & 4B.
[0051] In addition to the feedback loop described above for the
angular position signal 12, there are additional feedback paths
that are used to compensate for temperature effects, power supply
drift, amplifier offset of the position detector 14 and changes in
the parametric values of the resistors R1-R6, the switches SW1-SW6
and the torque coil 38. When these parametric values change, then
accordingly the values of the electrical signals of the channels
72-82.
[0052] Recalling that the measured value of acceleration is based
on calculating the total current through the torque coil, which is
proportional to the current signals of the channels 72-82 and their
respective duty cycles, if the current signals of the channels
72-82 change then the measured value of the acceleration changes as
well. Therefore in order to provide ongoing calibration of the
accelerometer, these changes must be tracked and compensated
for.
[0053] By providing the above multiple feedback paths, the digital
accelerometer of the present invention is able to track and
compensate for very low frequency signals, such as drift, while
simultaneously measuring the angular position signal 12, even when
the signal 12 is also a comparably low frequency signal.
[0054] Referring again to FIGS. 1A, 1B & 1B, there is a
calibration analog-to-digital converter 92, a temperature sensor 94
and a voltage divider 96. The temperature sensor 94 provides an
analog measurement of the ambient temperature in the form of a
temperature signal 98. The voltage divider provides a power supply
signal 100, which is a scaled version of a power supply
voltage.
[0055] The calibration analog-to-digital converter 92 converts the
temperature signal 98, the power supply signal 100, an amplifier
offset signal 102 and torque coil signals Cal- and Cal+ into
corresponding digital signals used by the algorithm, and in
particular the digital feedback network 32, to compensate the PWM
signals 46 and 48 for changes in these signals over time.
[0056] Referring again to FIG. 3, there is a block diagram of the
force-balance type accelerometer 11 and the negative feedback
network described above. A controller block 33 comprises the
multiplier 30 and the digital feedback network 32. A PWM generator
block 35 comprises the PWM generators 24 and 26. A three level
switch block 37 comprises the multiplexor 50, the switches SW1-SW8
and the resistors 72-82.
[0057] Referring now to FIGS. 7A & 7B, the operation of the
accelerometer, and more particularly the feedback signal 36, is now
described for a sampling period T.sub.SAMPLE. FIGS. 7A & 7B has
a first graph 84 illustrating the first PWM signal 46 when the duty
cycle is being controlled by the MSW full scale segment 42, a
second graph 86 illustrating the first PWM signal 46 when the duty
cycle is being controlled by the MSW segment 40, a third graph 88
illustrating the second PWM signal 48 and a fourth graph 90
illustrating a voltage across the torque coil 38. The graphs 84,
86, 88 and 90 illustrate two consecutive sampling periods
T.sub.SAMPLE. The horizontal axes of the graphs 84, 86, 88 and 90
represent time and the vertical axes represent voltage. The
horizontal axes of the graphs 84, 86, 88 and 90 of FIGS. 7A &
7B refer to the following stages: TABLE-US-00002 Stage Name A
PRELOAD B MSW (or MSW FULL SCALE) C MSW WAIT D COMPENSATION E WAIT
COMPENSATION F LSW G IDLE
[0058] The PWM cycle starts with a PRELOAD pulse at stage A that
charges the torque coil 38. The PRELOAD pulse is always present
regardless of the duty cycle. After the PRELOAD pulse, a MSW (or
MSW Full Scale) pulse is generated at stage B with a duration that
matches the MSW (upper fourteen or ten bits) of the digital
feedback signal 34. After the MSW pulse there is a MSW WAIT period
at stage C, which is initiated to allow the coil to discharge. The
MSW WAIT period duration is fixed.
[0059] After the MSW WAIT period, a COMPENSATION pulse is generated
at stage D to compensate for the presence of the PRELOAD pulse. The
COMPENSATION pulse is generated in the opposite direction of the
PRELOAD pulse. There is an additional WAIT COMPENSATION period
initiated at stage E to wait for the coil to discharge.
[0060] After the WAIT COMPENSATION pulse, a LSW pulse is fired to
generate the residual LSW at stage F. An IDLE period at stage G
after the LSW is variable in length and its purpose is only to add
up time up to the next cycle and to allow for the torque coil 38 to
discharge. A full period of the cycle T.sub.SAMPLE represents both
the sampling and the PWM update frequency.
[0061] It is an advantage of the present invention to use the
combination of the PRELOAD pulse and the equal but opposite
COMPENSATION pulse. The COMPENSATION pulse cancels the transient
energy delivered to the torque coil 38 caused by the PRELOAD pulse
activating the switches SW1 or SW2, thereby ensuring that the total
energy delivered to the torque coil 38 is controlled precisely by
the PWM signal 46 and 48, and therefore the digital feedback signal
34. The current signals of the channels 76 and 82 are relatively
small, in this example, and therefore not much transient energy is
created when switching on the switches SW3 and SW6.
[0062] Assuming that the pulse amplitude, i.e. the amplitude of the
current signals through the torque coil 38, is stable, its
compensation does not take place every cycle. The compensation
takes place either on a random basis or at a fixed period. The
pulse has to be of a certain duration to provide enough time for
the ADC 18 to complete the conversion. If the required pulse width,
at the time when amplitude measurement is to take place, is too
small, then the pulse width gets extended up to the point that the
analog to digital conversion may successfully complete. The extra
amount of time added to MSW (or MSW Full Scale) to allow for the
analog to digital conversion of the ADC 18 to complete is
compensated by the extended duration of the COMPENSATION pulse.
This technique entirely encapsulates the process of calibration
normally invisible from the data collection point of view.
[0063] The force generated by the torque coil 38 due to the
feedback signal 36 on the unbalanced weight 13 is in a direction
opposite to force created by the input acceleration, and rises in
value until the force generated matches the force of input
acceleration, i.e. the current through the torque coil 38 is
directly proportional to the acceleration. Thus, the acceleration
is directly obtained by reading the digital feedback signal 34,
i.e. the PWM duty cycle value, which actually is the output of the
system.
[0064] The feedback signal 36 has a significant amount of high
frequency components related to the PWM switching that are
eliminated by a low pass filter, preferably an FIR type, to provide
a linear phase response.
[0065] In other embodiments, it is possible to overlap the MSW
stage (or the MSW FULL SCALE stage) and the LSW stage. As an
example, this is possible when the electrical signals of the
channels 72-80 are provided by current sources. In this situation,
the electrical signals are added together and applied to the torque
coil 38.
[0066] Referring to FIGS. 1A, 1B, 1C & 3, there is an output
filter block 106 which receives as an input the digital feedback
signal 34. The digital feedback signal 34 is an extremely accurate,
controlled feedback signal that may be used for other purposes. The
output filter removes high frequency components introduced as a
result of switching on and off the switches SW1-SW6.
[0067] Referring now to FIGS. 8A, 8B, 8C and 8D, there is shown a
schematic diagram of the embodiment of FIGS. 1A, 1B and 1C.
[0068] As apparent to those skilled in the art, various
modifications can be made to the above described solution within
the scope of the appended claims. For example, the resistors R1,
R2, R3, R4, R5 and R6 can be replaced by current sources, or by the
resistors of different values to allow for different scale factors.
By varying the bandwidth of the low pass FIR filter the overall
system bandwidth can be changed to allow for different dynamic
response and/or output noise level (dynamic resolution). Also, the
output FIR filter can be replaced by a more sophisticated
estimator, such as Kalman regulator, to further reduce the amount
of noise. Moreover, the output FIR filter can be removed from the
circuitry to allow for non-real time signal processing to
facilitate use of more sophisticated and complex algorithms
normally not deployed in real time due to the required processing
power. The current distribution (ratio of the currents in the
channels 72, 74 and 76 and in channels 78, 80 and 82) can be
achieved in many different ways to allow for different
implementation of the current source, duty cycle, sampling
frequency, etc. The proposed solution relates to a single axis
acceleration measurement but the circuit can be easily modified to
obtain two- and triaxial acceleration measurement. In that case, a
single DSP with six channel PWM generators can be used to
simultaneously drive three double level PWM signals for three
channels through three different multiplexers and three H bridges.
The discussed PWM drive can also be applied in different
applications wherever a high resolution D/A conversion in form of
PWM is required.
[0069] As will be apparent to those skilled in the art, various
modifications may be made in the above-described embodiment of the
present invention within the scope of the appended claims.
* * * * *